1. Field of the Invention
The invention is related in general to systems and methods involving the use of magnetic field sensors, and in particular the invention is related to systems, methods, and apparatus involving sensors and circuits that cancel magnetic field noise while measuring torque-induced magnetic fields.
2. Description of the Related Art
U.S. Pat. No. 5,351,555, the disclosure of which is incorporated herein by reference in its entirety, discloses a single circularly magnetized region in which the magnetic dipoles tilt in the presence of torsional stress, thereby emanating an externally measurable magnetic field. Because magnetic fields, in the context of their measurement, are fungible, the sensor taught in the '555 patent may be susceptible to other magnetic fields of exterior origin. In particular, the Earth's magnetic field will cause a phenomenon known as “compassing,” in which the measured field is the sum of the torque dependant field plus the Earth's north-south magnetic field component. Within the context of this disclosure, the term “compassing” shall be used to describe any error resulting from interaction between the magnetic field sensors and magnetic fields of external origin.
U.S. Pat. No. 5,520,059, the disclosure of which is also incorporated herein by reference in its entirety, addresses the compassing issue with the addition of an adjacent second region that is magnetized in the opposite circular direction to the first region. This arrangement yields two torque-dependent magnetic fields and, because the acquiescent magnetization of the regions is in opposite directions, the torque-dependent magnetic fields are of equal but opposite magnetic polarity. Corresponding with the two regions described in the '059 patent are two magnetic field sensors, each with an opposite axial polarity to the other (but with the same polarity relative to each of the corresponding magnetized regions). Thus, an ambient magnetic far field affects each of the field sensors in an equal but opposite manner, thereby canceling its measurement. That is, a non-divergent (far) field would affect each of the corresponding field sensors with approximately equal magnitude, but with opposite polarity (owing to their installed configuration); thus by summing the outputs all common mode external magnetic fields would be cancelled.
While the teachings of the '059 patent are effective when dealing with far fields, a divergent near field can expose each of the two magnetic field sensors to distinctly different field intensities and direction. In this scenario, the two field sensor outputs will not reflect equal but opposite error components that cancel each other, but rather unequal and opposite components that introduce an error to the measurement. In practice, the configuration of the invention disclosed in the '059 patent is error-prone in the presence of locally divergent magnetic fields because the two magnetic field sensors experience different magnitudes of the divergent magnetic fields. The difference in magnetic fields between the two magnetic field sensors originating from a near field source combines non-uniformly with torque induced magnetic fields and leads to a false torque value. Thus, it is important to eliminate this near field effect.
There are numerous other types of near field sources that can compromise an accurate torque-dependent field measurement. These sources include a permanent magnet, a magnetized wrench, a motor or solenoid, etc. Another would be the nearby presence of a ferromagnetic structure that distorts the shape and direction of the earth's magnetic field, creating a localized area in which the magnetic flux is concentrated in an undesirable direction. Each of these examples results in a divergent magnetic field, i.e., one in which there are significant local gradients in both magnetic field strength and flux direction.
There are numerous methods for canceling the effects of near field source or stray magnetic fields. These include employing shielding and using flux directors. Each of these types of structures is made from materials having a high magnetic permeability, meaning that they present a much lower resistance to magnetic fields than, for example, air. In principle, a shield would be in the form of a tube of infinite length, although shorter finite lengths may suitably function. Magnetic fields originating outside of the shield are effectively shunted through the highly permeable shield material, which prevents them from intersecting the field sensors. Using a different approach, a flux director “gathers” most of the torque dependent magnetic field and directs it into the magnetic field sensors. With this approach, the flux director geometry is such that its effectiveness of gathering the torque dependent magnetic field of interest is much greater than its effectiveness of gathering extraneous and error inducing magnetic fields, thus increasing the efficiency of the magnetic field sensors and hence, their signal to noise ratio.
While the shielding method noted above can be effective for external magnetic fields perpendicular to the axial direction of a shield in the form of a tube, this shield is very vulnerable to external magnetic fields in the axial direction of the tube which is open at both ends. Any external magnetic fields can transfer to the field sensors inside the shield through the sides of the shield which are open.
Flux director structures typically operate by gathering the radial flux component of the torque dependent magnetic field, and are therefore well suited for rejecting axially directed flux of external origin, however, flux directors tend to be susceptible to external fields perpendicular to the axis of the shaft.
A combination of tubular shielding and flux directors would act in a complimentary manner by effectively mitigating both axially and radially directed fields of external origin acting directly on the field sensing devices. However, such a combination has other shortcomings that limit its desirability in many applications including cost and packaging within the design.
If an external magnetic field source is directly contacted with the end of a shaft such as the end of the column of an electric power steering system, a strong external near field could transfer to the field sensors through the shaft as a result of diametric variations in the shaft or nearby magnetically coupled structures such as, for example, a bearing or mounting flange. Moreover, a typical manufacturing process for a column or shaft may include a magnetic particle inspection (MPI) process that involves a magnetization process for guiding magnetic particles into the defect sites for visualization of defects on column surface, and a demagnetization process after finishing the inspection. Frequently, demagnetization is not perfect, and there remains a remanent magnetic field in the column or shaft after the MPI process. Typical values of the remanent magnetic fields are between 10 and 100 Gauss. This relatively large external magnetic field can be directly transferred to the field sensors inside the shield, and can be non-uniformly summed with the torque-induced magnetic fields, corrupting the torque measurement. This means that there is no totally effective way to protect or shield external magnetic fields propagating through the shaft with current techniques.
An additional disadvantage of the shielding method is that any deformation of the shield device caused by mechanical impact or extreme temperature change can affect the relative position of the field sensors and the shield, which can lead to unbalancing of far field values between two sensor fields operating in pairs that are oppositely oriented. This would result in compassing failure.
Furthermore, in most torque sensor applications, packaging space is limited, and in many cases there is no room for a shield or flux director. In addition, the added financial cost for those components is not insignificant because materials with high permeability tend to have high percentages of nickel, the pricing of which is quite volatile.
Based on the foregoing, there is a need for a new and better technique for effectively canceling the effects of non-torque dependent magnetic fields without using shielding materials or flux directors. The present invention meets these requirements by special arrangement of field sensors so as to effectively eliminate or minimize measurement error resulting from divergent near fields without using shielding materials and flux directing devices.